1. Field of the Invention
The present invention relates to a magnetically inductive flow meter unit for determining a flow rate of a fluid flowing through a measuring tube.
2. Description of the Background Art
Magnetically inductive flow meters are known and are based on the principle of separating charged, moving particles in a magnetic field. A magnetically inductive flow meter thus has a magnetic field perpendicular to the direction of flow of the fluid to be measured. The charged particles contained in the fluid are deflected and separated by the applied magnetic field perpendicularly to the magnetic field. A voltage induced by the separated, charged particles may be tapped with the aid of two measuring sensors disposed perpendicularly to the magnetic field. The tapped voltage is a measure of the flow rate.
A distinction is made between a galvanic and a capacitive signal tap. In measuring sensors for a galvanic signal tap, the electrode of the measuring sensor is in direct contact with the fluid. As a result thereof, the electrode is exposed to contamination and damage. For example, sand or rust particles contained in a fluid such as water may contaminate or damage the surface of the electrode.
For this reason, it is proposed in WO 85/04954 A1 to dispose a cap made of a porous, ceramic material, glass or a porous plastic on the surface of the measuring electrode, so that, while the fluid to be measured is able to penetrate all the way to the electrode, the particles contained in the fluid are held back by the cap.
Such measuring sensors having a cap on the surface of the electrode deliver imprecise, namely excessively low, measured values in a magnetically inductive flow meter, in particular at low flow rates to be measured. Thus, magnetically inductive flow meters having the aforementioned type of measuring sensors frequently do not display a flow rate of a fluid such as water, even though a low flow rate of the fluid is present. It has been furthermore established that the particles contained in the fluid produce a lot of noise on the electrode upon striking the electrode. The measuring signal at the electrode is thus significantly reduced by the upstream cap, which, in turn, results in a lower, i.e., poorer, signal-to-noise ratio.